1. Field of the Invention
The present invention relates to an imaging apparatus which, more particularly though not exclusively, can be used in teleconference systems and remote monitoring systems.
2. Description of the Related Art
In a typical electronic camera, a subject image formed on an image pickup device, such as a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) sensor is converted into an electric signal and further converted into a digital signal. Then, the resultant digital signal is subjected to predetermined signal processing, such as YC processing, and output as an image signal of a predetermined format.
FIG. 11 is a block diagram showing a main configuration of a typical camera system.
In FIG. 11, reference numeral 101 denotes a fixed front lens (first lens unit), reference numeral 102 denotes a zoom lens (second lens unit) driven by a stepping motor 117, reference numeral 103 denotes an iris (or diaphragm) driven by a motor 118, reference numeral 104 denotes a fixed lens (third lens unit), and reference numeral 105 denotes a focus lens 105 driven by a stepping motor 119. Reference numeral 150 denotes a temperature sensor. Reference numeral 106 denotes an image pickup device, such as a CCD or a CMOS sensor, reference numeral 107 denotes a correlated double sampling and automatic gain control (CDS/AGC) circuit, reference numeral 108 denotes an analog-to-digital (A/D) converter, reference numeral 109 denotes a signal processing circuit, reference numeral 110 denotes a digital-to-analog (D/A) converter 110, and reference numeral 111 denotes a memory (storage device). Reference numeral 112 denotes an infrared cut filter for eliminating undesired infrared components. Reference numeral 113 denotes a controller (microcomputer) and reference numeral 114 denotes a power rotating platform capable of panning and tilting to any desired angle as illustrated in FIG. 2. Reference numeral 115 denotes a processing circuit for detecting autoexposure/auto white balance (AE/AWB) evaluation values. Reference numeral 116 denotes a processing circuit (hereinafter referred to as “AF-evaluation-value detecting circuit”) for detecting an autofocus (AF) evaluation value. Reference numeral 120 denotes a timing generator.
In the configuration described above, the lenses 101, 102, 104, and 105 converge light from a subject to form an image on an imaging plane of the image pickup device 106. The image is then converted into an electric signal (analog video signal). The analog video signal passes through the CDS/AGC circuit 107 and is input to the A/D converter 108. When the iris 103 is in the full open position and the luminance signal level (the amount of light received by the image pickup device 106) is below a predetermined value, the CDS/AGC circuit 107 amplifies the analog video signal according to the brightness of the subject. Next, the A/D converter 108 converts the analog video signal into a digital signal (digital video signal). The signal processing circuit 109 performs processing that meets the video signal standard on the digital video signal. Examples of such processing include color separation, white balance control, and gamma correction. After the processing, the digital video signal is converted by the D/A converter 110 into a video signal of an appropriate format and output.
The AF-evaluation-value detecting circuit 116 includes a gate circuit for gating video signals corresponding to an area within a predetermined auto-focus frame defined in the imaging plane. The AF-evaluation-value detecting circuit 116 further includes a band-pass filter (BPF) for extracting, from the gated video signals, high-frequency components necessary for in-focus detection. A sharpness (focus evaluation) signal detected by the AF-evaluation-value detecting circuit 116 is supplied to the controller 113, which controls functions of the overall system, including AF, AE, and AWB.
The imaging plane of the image pickup device 106 includes color filters for producing images of respective colors. The infrared cut filter 112 for eliminating undesired infrared components is placed in the imaging optical path. A motor 121 is provided for insertion and removal of the infrared cut filter 112. Under low illumination, the motor 121 removes the infrared cut filter 112 from the optical path to improve sensitivity using the infrared sensitivity of the image pickup device 106. Directing infrared light from an infrared projector to the subject allows shooting under even darker conditions.
However, the insertion and removal of the infrared cut filter 112 affect the focus position. In other words, since the focus position changes depending on whether the infrared cut filter 112 is present in the optical path, it can be necessary in some circumstances to compensate for changes in back-focus position.
FIG. 12 shows changes in zoom lens position and focus lens position, in the above-described camera system (typical rear-focus zoom lens system), with respect to a subject at infinity under a normal light source (e.g., 550 nm). Referring to FIG. 12, curve (a) represents the change when the infrared cut filter 112 is placed in the optical path, and curve (b) represents the change when the infrared cut filter 112 is not placed in the optical path. A focus difference indicated by “h” in FIG. 12 corresponds to the amount of back focus change. As shown, an in-focus position with respect to the subject at infinity is different depending on whether the infrared cut filter 112 is placed in the optical path.
FIG. 13 illustrates changes in sharpness signal (focus evaluation signal) output during shooting of the same subject (at infinity) under a normal light source. Curve (a) represents the change when the infrared cut filter 112 is placed in the optical path, and curve (b) represents the change when the infrared cut filter 112 is not placed in the optical path.
Japanese Patent Laid-Open No. 2002-221656 discusses a technique for compensating for changes in in-focus position, and specifically discusses a technique for varying the driving range of a focus lens depending on whether an infrared cut filter is present in the optical path.
However, typically only a back focus compensation is performed, thus significant defocusing can occur during a zooming operation.